Ultrasound imaging systems for medical applications typically employ an array of individual ultrasound transducer elements which transmit and receive ultrasound energy. The transducer array transmits ultrasound energy into a region of interest in a patient, and receives reflected ultrasound energy, or echos, from various structures and organs within the patient's body. The imaging system then processes electronic signals generated by the elements of the transducer array, based on the received ultrasound energy, to form an image of the region of interest.
Some ultrasound imaging applications make use of harmonic reflections from a region of interest for which an image may be desired. Harmonic reflections may result from a non-linear medium that is exposed to ultrasound energy at some fundamental transmit frequency. One example of such a non-linear medium includes water, which is present throughout body tissues and has different expansion and compression properties upon exposure to ultrasound energy. In this manner, non-linear body tissues and fluids may release acoustic energy at one or more harmonic frequencies of the fundamental transmit frequency of the ultrasound energy to which they are exposed.
Contrast agents provide another example of a non-linear medium used in some ultrasound imaging applications. Non-linear contrast agents may be introduced into regions of interest in a patient, for example, into the blood stream or body tissues, to highlight these regions from surrounding tissue in the ultrasound image. These agents generally have stronger non-linear properties than the surrounding tissues. Typically, contrast agents have a fundamental resonant frequency and radiate ultrasound energy at a particular harmonic frequency when exposed to high intensity ultrasound energy at the fundamental resonant frequency. An ultrasound imaging system may therefore identify and isolate regions containing a contrast agent by differentiating the ultrasound energy at the particular harmonic frequency associated with the contrast agent from harmonic ultrasound energy associated with the surrounding tissue.
Ultrasound imaging applications utilizing harmonic reflections from either body tissues and/or contrast agents may be limited, however, by the harmonic content of ultrasound energy transmitted to a region of interest. In particular, ultrasound transmit waveforms having a significant harmonic content may result in ultrasound images having undesired artifacts, as well as ultrasound images having reduced contrast between a contrast agent and surrounding tissues. Such artifacts or reduced contrast may be due to undesired high frequency components of the transmit waveform that interfere with desired harmonic reflections from the region of interest.
For example, to reach a region of interest for which an image may be desired, ultrasound energy must often pass through body structures and tissues, such as a chest wall, which include inhomogeneous materials that may significantly distort the waveform profile of ultrasound energy. This distortion from inhomogenous materials often results in unwanted artifacts in the resulting image. Since ultrasound waveform distortion from inhomogeneous materials is typically less at lower frequencies, it is advantageous to transmit ultrasound energy at a low fundamental frequency. However, lower frequency ultrasound energy generally results in lower resolution images.
To avoid a loss of resolution associated with lower frequencies, an ultrasound imaging system may detect harmonics reflected from any non-linear tissues being imaged, as discussed above. However, if the ultrasound transmit waveform itself includes a significant harmonic content, higher frequency components of the transmit waveform may not be discernible from the desired harmonic reflections. Moreover, distortion of the higher frequency components of the transmit waveform from inhomogeneous materials, notwithstanding the lower fundamental frequency, may contribute to unwanted artifacts in the resulting image.
In view of the foregoing, it is desirable to transmit ultrasound energy as a waveform having a fundamental frequency, wherein the waveform is substantially free of higher frequency harmonic components of the fundamental frequency, or has low harmonic content. Since higher frequencies experience greater distortion in inhomogeneous materials, as discussed above, reducing higher frequency components in an ultrasound transmit waveform in turn reduces distortion of the ultrasound energy as it passes through body structures and tissues that include inhomogeneous materials. By utilizing ultrasound transmit waveforms having low harmonic content, it is possible for ultrasound imaging systems to obtain clearer images having fewer artifacts from distortion. Additionally, ultrasound imaging applications utilizing low harmonic content ultrasound transmit waveforms and contrast agents benefit from a higher contrast ratio between the contrast agent and surrounding tissue.
In one known technique for reducing the harmonic content of an ultrasound transmit waveform, an electronic pulse generator supplies a burst of pulses having a fundamental frequency to a low pass filter. The duration of the pulses and the number of pulses in the burst are controllable and determine the resulting frequency spectrum of the burst. Such a burst of pulses has a primarily rectangular shape and has a frequency spectrum that includes a number of sidelobes having significant harmonic content. The low pass filter is typically designed as a Gaussian, Bessel or Chebyshev filter to substantially eliminate energy at a particular harmonic frequency from the burst of pulses. The filtered burst of pulses is then applied to ultrasound transducer elements which transmit ultrasound energy having a waveform similar to that of the filtered burst of pulses.
Other proposed techniques for reducing the harmonic content of ultrasound transmit waveforms include using digital signal processing methods and apparatus, such as digital programmable waveform generators, to produce particularly shaped electronic waveforms having low harmonic content, which are then applied to ultrasound transducer elements. Such techniques typically involve synthesizing an electronic signal having a particularly shaped amplitude envelope which includes several cycles of a fundamental or "carrier" frequency. A variety of such electronic signals having frequency spectra that exhibit "sidelobe suppression," or low harmonic content, may be custom synthesized using digital signal processing methods and apparatus.
In one such technique described in Hossack, et al., U.S. Pat. No. 5,740,128, a desired frequency spectrum for an ultrasound transmit waveform is designed on a computer, and an inverse fast Fourier transform is performed to synthesize a corresponding time domain waveform. The synthesized waveform may be specifically designed to suppress ultrasound energy in a wide band around a particular harmonic frequency. A digital representation of the synthesized waveform is stored in memory, and is applied to a digital-to-analog converter which provides an analog electronic signal of the stored waveform. This analog signal may optionally be passed through a low pass analog filter, such as a Gaussian, Bessel filter or a Chebyshev filter, to further reduce any undesirable high frequency components of the analog signal. This low harmonic content electronic signal is then applied to ultrasound transducer elements to provide ultrasound energy having low harmonic content.
One consequence of the techniques described above is that a significant amount of space is required for the electronic circuitry, for example, the analog filters or any digital signal processing electronics, such as a computer or programmable waveform generator, memory, and digital-to-analog converters, which provide the low harmonic content electronic signal applied to the ultrasound transducer elements. Accordingly, a method and apparatus for transmitting ultrasound energy having low harmonic content is desirable that uses fewer electronic components and requires less space than other proposed techniques.